1. Field of the Invention
The present invention relates to heat recovery. In particular, the present invention relates to an apparatus and method for recovering heat from wastewater.
2. Discussion of Background
Many industrial and domestic processes require the maintenance of a stable temperature in a stream of flowing liquid, despite the fact the new material is constantly entering and old material flowing out. This is often accomplished by blending two incoming streams, one above and one below the desired temperature, using proportioning valves.
In industrial processes, waste heat can be recovered from the outgoing stream and returned to the incoming low-temperature stream by a heat exchanger. Since some of the heat is unavoidably lost, the incoming high-temperature stream must make up the heat deficit. The flow rate of the high-temperature stream is adjusted by an automatic temperature controller to maintain equilibrium at the desired temperature.
Arrangements for domestic use are usually much simpler. Proportioning is done through hand-operated valves such as water faucets, with the temperature adjusted simply by touch. No effort is made to recover the heat which remains in used wash water. This not only wastes energy and money, but also makes the temperature more liable to sudden change if the pressure in the supply lines fluctuates.
The perceived benefit of industrial heat exchangers and temperature controllers is generally thought to be inadequate to justify their cost in a home plumbing system. However, this issue is not as clear-cut as it appears.
Suppose that a typical household contains two people who each shower once a day, using five gallons of water per shower. Suppose, further, that water enters the home at an average temperature of 70.degree. F. (21.degree. C.) and the shower water is used at 110.degree. F. (43.degree. C.). Heating 10 gallons of water per day by 40.degree. F. (22.degree. C.) requires about 25,000 BTU, or, in an all-electric household, 7.3 KWh. This amounts to 9.1 million BTU, or 2,700 KWh per year. At typical costs per KWh, anywhere from 30%-50% of the total cost of operating a 40-gallon hot water heater is spent on producing heated shower water.
Family members (or appliances such as a dishwasher or an icemaker) sometimes turn on water valves or faucets unexpectedly while others are showering. The flow through a valve is determined not only by the valve setting, but also by the pressure difference across it. Shower-water temperature is set by empirically balancing the faucet settings based upon the pressures existing at the time of setting. Sudden pressure changes can upset the balance, causing uncomfortable temperature shifts. At worst, painful scalding can result. A temperature shift of another sort, seldom dangerous but always aggravating, occurs when the water heater runs out of hot water in mid-shower.
A wastewater heat recovery system suitable for domestic or industrial use would be reasonably efficient, would include a temperature controller, require a minimal maintenance, and would conserve energy and thereby save a substantial portion of a typical household's water-heating costs. It would also contribute to home comfort and safety. Such a system would also be inexpensive and have a short payback period. An installation cost comparable to 1-2 years' energy savings could readily be justified on economic grounds alone. A somewhat higher cost might be allowable if comfort and safety were also taken into account.